1. Field of the Invention
This invention relates to the field of micro-electromechanical (MEM) devices, and particularly to MEM switches and their applications.
2. Description of the Related Art
Many circuits require a multiplexing function, in which an incoming signal is selectably switched to one of N output terminals. This is commonly accomplished with electromechanical or solid-state switches—typically field-effect transistors (FETs)—which are closed as necessary to provide the desired signal path.
However, there are several problems related to the use of solid-state switches, particularly at very high frequencies. Integrated switches capable of handling such frequencies are typically implemented with gallium arsenide (GaAs) MESFETs or PIN diode circuits. At high signal frequencies (above about 900 MHz), these switching devices or circuits typically exhibit an insertion loss in the ON (closed) state of about 0.5 db. Additional gain must often be built into a system to compensate for the poor performance of the devices, increasing power dissipation, cost, and unit size and weight. The characteristics of GaAs MESFETs and PIN diode switches are discussed, for example, in R. Dorf, The Electrical Engineering Handbook, CRC Press (1993), pp. 1011–1013.
Providing switching with PIN diode circuits presents additional problems due to the parasitic capacitances inherently created by their use, which serve to limit the frequency range over which the circuit can operate. Similar problems arise when the necessary switching is provided by off-chip switches, due to the parasitic capacitances that result from the presence of wire bonds.
Another approach requires the use of micro-electromechanical (MEM) switches. MEM switches generally provide lower insertion losses than MESFETs or PIN diode circuits, and are particularly well-suited to use with very high frequency signals. MEM-based multiplexers might be used, for example, for switch matrices, component selection, signal routing, redundancy switching, or to implement a multi-bit phase shifter. An example of a 2-bit phase shifter circuit is shown in FIG. 1. An incoming signal is applied to an input terminal IN, and is switched to a 0° or 90° delay circuit via respective MEM switches 10 and 12 which form a 1:2 multiplexer 13. The outputs of the 0° and 90° delay circuits are provided at a node 14 via another 1:2 mux 15 comprising MEM switches 16 and 18, which are needed to isolate the signal path from unused transmission line sections. A mux 19 comprising MEM switches 20 and 22 switch node 14 to 0° or 180° delay circuits, respectively, the outputs of which are provided to an output terminal OUT via a mux 23 comprising MEM switches 24 and 26, respectively.
This approach can also prove troublesome, however. Switched signals can be subject to insertion losses due to inductance mismatch and signal reflection on the multiplexers' output lines. This is particularly bad for an application such as the 2-bit phase shifter shown in FIG. 1, in which an incoming signal must pass through four MEM switches before reaching the output terminal. Another drawback is that each multiplexer requires a considerable amount of die area.